This invention relates to spark plugs and more particularly to a spark plug and connector assembly in which the spark gap is defined by two electrodes which are both electrically isolated from ground.
Spark plugs for internal combustion engines generally consist of an insulator mounted within a shell. The shell has a threaded end adapted to engage an opening leading through the head of the engine and into a combustion chamber. A center electrode is mounted within a bore through the insulator and forms a spark gap with a ground electrode attached to the shell. During operation of the engine, a high-voltage ignition system periodically applies high-voltage pulses to the center electrode. The resulting arc across the spark gap ignites a fuel-air mixture within the combustion chamber.
In certain engines, it is desirable to provide two spark plugs for each combustion chamber. Ideally, high-voltage pulses are applied simultaneously to the two spark plugs for initiating combustion at two spaced points within the combustion chamber. By providing two separate sparks, the time required for the flame to propagate across the combustion chamber is decreased and also the reliability of the engine is greatly increased. These benefits are of particular value in engines operated with lean fuel-air mixtures. Generally, two separate ignition systems have been provided, one for firing each of the two spark plugs for each combustion chamber. However, difficulty has occurred in timing the operation of the two spark plugs to fire simultaneously. Also, the use of two separate ignition systems adds considerably to the cost of the engine.
It has been suggested that a single ignition system be adapted to simultaneously fire two spark plugs in an internal combustion engine. One suggestion was to connect two conventional spark plugs in parallel between the high-voltage ignition cable and electrical ground. However, the two spark plugs will not have identical characteristics. During operation of the engine, one of the two spark plugs will fire at a voltage lower than the other spark plug and, as a consequence, only the one spark plug will fire. Another suggestion has been to connect the two spark plugs electrically in series. With this arrangement, currents will flow in series across both spark gaps and, therefore, both spark plugs must fire simultaneously. However, two conventional spark plugs cannot be connected in series since one electrode on each spark plug is grounded to the shell. It has been found that by designing a spark plug with two electrodes which are both electrically insulated from ground, a system may be constructed in which two series connected spark plugs are simultaneously operated from a single high voltage ignition system. The high-voltage ignition cable from the ignition system is connected to one of the two non-grounded electrodes. The other of the two electrodes is connected by means of a high-voltage cable to the center electrode of a conventional spark plug. As a consequence, a circuit is formed from the high-voltage ignition system through the spark plug having the two insulated electrodes and then through a conventional spark plug to electrical ground.
Early prior art suggested designing spark plugs with two electrically insulated electrodes for forming a spark gap. In one such spark plug, as shown in U.S. Pat. No. 1,217,784 which issued Feb. 27, 1917 to LaPlante, two electrically isolated electrodes are mounted within the spark plug insulator. However, one of the two electrodes is then connected to ground and the other is connected to a high-voltage ignition cable by means of a bolt terminal. Another similar type of spark plug is shown in U.S. Pat. No. 1,560,512 which issued Nov. 3, 1925 to Hirsch. Again, two electrically isolated electrodes are mounted in the insulator of a spark plug. High voltage is applied to the spark plug by means of bolt terminals. The Hirsch patent discloses the use of the spark plug in a series electrical circuit. However, difficulty occurs in making electrical connection to the Hirsch spark plug. The primary consideration in the La Plante spark plug was to provide a spark plug having an electrode configuration for reducing fouling during operation. Difficulty also occurred in making electrical connection to the La Plante spark plug.